(1) Field of the Invention:
The present invention relates to a method and apparatus for adjusting the toner concentration a two-component type developer. More particularly, the present invention relates to a method and apparatus for precisely and freely adjusting the toner concentration within a broad range on a magnetic brush in which a two-component type magnetic developer is actually used for the development.
(2) Description of the Prior Art:
In the electrophotographic process or electrostatic recording process, an electrostatic latent image is formed on a photosensitive material or recording material and this electrostatic latent image is brought into contact with a developer to form a visible image corresponding to the electrostatic latent image. For the development of the electrostatic latent image, a two-component type developer comprising a magnetic carrier composed of iron powder and an electroscopic toner comprising resin binder particles containing a pigment dispersed therein in widely used because the development operation is facilitated with the use of this two-component type developer. If these two components of the developer are mixed, the toner particles are frictionally charged with an inherent charge and they are held on the surface of the magnetic carrier. If this two-component type developer in the form of a magnetic brush is brought into sliding contact with a substrate bearing an electrostatic latent image, the toner particles are attracted to the electrostatic latent image to form a toner image.
The two-component type developer is advantageous in that the development operation is facilitated as described above and the quality of the formed image is ordinarily good. However, the two-component type developer is defective in that while the development operation is continued, the toner concentration in the developer is gradually reduced and it is difficult to adjust the toner concentration at a certain level. More specifically, with reduction of the toner concentration in the developer, the density of the formed image is correspondingly reduced. On the other hand, if the toner concentration is too high, such troubles as adhesion of the toner to the background (non-image area) and scattering of the toner from a magnetic brush roller are caused.
Accordingly, reduction of the toner concentration in the two-component type developer is detected according to reduction of the image density, change of the inductance of the developer or change of the volume of the developer, and the toner is manually supplied. However, when the toner is manually supplied, the toner concentration is changed stepwise and it is difficult to adjust the toner concentration in the developer at a certain predetermined level. There is known an apparatus in which the toner concentration is detected and the toner is automatically supplied based on the detection signal. However, a complicated detection and control system is necessary for this apparatus and the apparatus is inevitably expensive, and also in this apparatus, the adjustment of the toner concentration is inevitably stepwise.
Furthermore, in this known toner concentration adjustment mechanism, if the toner concentration in the two-component type developer becomes higher than the predetermined level, the function of reducing the toner concentration cannot be attained.